This invention relates to methods and devices for measuring the body temperature of a patient in conjunction with the placement within the patient of an access device, for example, a catheter or introducer.
The needs to properly treat a patient and to gain as much information as possible about the physiological state of a patient are often at odds with the desire to reduce discomfort to the patient as much as possible. For example, there is frequently a need both to deliver various medications to a patient, and also to monitor the patient""s body temperature. Accordingly, catheters are often inserted into the vasculature of a patient to allow delivery of various medications, hydrating fluids, etc., and to measure blood pressure. The patient""s body temperature, however, is monitored with a separate device, which is inserted separately.
Conventional devices for measuring temperature include the well-known oral thermometer, rectal, axillary (armpit), and tympanic (ear) thermometers and probes, as well as Foley catheters (bladder temperature), and nasopharyngeal probes (esophagus) probes. Each of these devices suffers from one or more shortcomings. The first disadvantage is obvious to anyone who has ever been the patient: It is uncomfortable enough to have a catheter inserted into one""s vein or artery without also having to have a separate device inserted into one""s rectum, bladder, ear or nose, or down one""s throat.
The second disadvantage has to do with accuracyxe2x80x94taking a patient""s temperature by placing a thermometer under her armpit or in her mouth may cause the least discomfort to the patient, but the temperature value this provides is usually less accurate and much more dependent on placement than temperature measurements of blood in a major vessel.
One way to overcome these disadvantages is to include some form of temperature sensor within the inserted catheter itself. This allows for measurement of the blood temperature, which is in most cases much closer to the patient""s actual body core temperature. The problem then arises that other elements of the catheter system may have thermal properties that themselves affect the temperature that the sensor senses. This problem arises in the context of thermodilution systems for measuring cardiac flow. U.S. Pat. No. 4,817,624 (Newbower, Apr. 4, 1989), U.S. Pat. No. 5,176,144 (Yoshikoshi, Jan. 5, 1993), and Published European Patent Application 0 357 334 B1 (Inventors: Williams, et al., Mar. 7, 1990) for example, describe such systems. As is well known, in such a thermodilution system, the temperature of the cardiac blood flow is modulated according to a predetermined pattern that is created by the injection of an indicator, which is usually either a series of boluses of a relatively colder fluid, or heat. The downstream response to the temperature modulation is sensed by a thermistor and is used to calculate and estimate blood flow.
In systems such as Newbower""s, temperature modulation is accomplished by cooling the blood through precisely dosed boluses of a thermally well-controlled fluid colder than the blood. In Williams, modulated cooling of the blood is accomplished using a heat exchange mechanism that does not require actual injection of any bolus into the blood stream. In systems such as Yoshikoshi""s the blood is instead heated locally using a heating element that is mounted near the far (distal) end of a cardiac catheter. As before, a thermistor senses the downstream response profile, whose characteristics are used to calculate cardiac flow.
Such thermodilution systems have certain clinical limitations, since they must deal with several problems specific to this application. First is the problem of retrograde flow: If the thermistor is located proximal to the heater or bolus injection port, then the heated/cooled blood will flow back over the catheter tip. The temperature of the catheter itself, which may contain various other lumens, injectates, control wires, etc. can then affect the temperature profile of the thermally modulated blood and degrade the flow calculations.
To overcome this effect, the injection is replaced by a continuous infusion of indicator in order to obtain a new steady-state baseline; however, this is an undesirable clinical limitation due to the volume-loading the patient. Even when the thermistor is located distal relative to the heater/bolus port, this problem may still arise.
These thermodilution system catheters normally have a distal infusion lumen that passes beneath the thermistor or temperature sensor and exits at the tip of the catheter. Since the flow in such an infusion lumen can severely degrade the accuracy of the temperature sensor measurements, the flow is limited to a maximum amount in order for the blood flow measurement to still be accurate. Of course, such a limitation on infusion lumen flow is also undesirable from the clinical perspective.
An analogous problem of insulation arises in other cardiac devices as well, such as the catheter-based cardiac ablation system described in U.S. Pat. No. 5,688,266 Edwards, et al., Nov. 18, 1997). In Edwards"" system, an ablation electrode is used to kill tissue locally using heat, and one or more temperature-sensing elements are used to sense the temperature of the tissue to be ablated and allow precise control of the ablation temperature and time. Isolation, provided primarily by physical separation, is thus required between the electrode and the temperature sensors; otherwise, the sensors will tend to give readings that are too high.
At least one factor limits the use of these known systems in general use for measuring a patient""s body temperature: These systems are not arranged to measure the patient""s actual, natural body temperature at all, but rather the temperature of blood or some body tissue whose temperature the system itself has deliberately altered.
There are other devices, such as central venous catheters (CVC), peripheral catheters, and other catheter-like instruments such as introducers. As their names imply, such catheters do not require placement into the heart and are consequently used more frequently in different areas of the hospital. Unlike cardiac catheters, which are often more than 100 cm long and require an introducer for insertion, these devices are seldom longer than about20-30 cm and can be inserted by the Seldinger technique. A CVC, for example, is often placed in a patient""s jugular vein and is used for various infusions, for monitoring blood pressure, etc., through a number of lumens within the device.
An instrument such as a CVC often includes several different lumens which may carry a range of fluids (such as medications and other infusions), as well as instruments such as pressure transducers. Each of these fluids and instruments may be at different temperatures, or may have varying thermal properties, or both. Any measurement of temperature using such a catheter would thus risk serious thermal contamination from other portions of the catheter.
There are at present no known devices such as a CVC, peripheral catheter, or introducer that incorporate an arrangement for measuring blood temperature accurately. Therefore, it would be advantageous to be able to accurately measure temperature in conjunction with such access devices as catheters and introducers while eliminating the need to insert a secondary device into the patient in order to measure temperature, as is the current practice. Such devices would also provide a more accurate and less time-consuming body temperature measurement than non- or less invasive devices. This invention provides such an arrangement.
It would also be advantageous to be able to connect a CVC or similar catheter to a standard patient monitor. Not only would this bring the obvious benefit that the patient""s temperature could be viewed at a glance along with other monitored parameters, but it would also make the temperature values available for other processing as needed. Many patient monitors, however, use a signal standard that is compatible with large thermistors or temperature sensors and not compatible with the output of miniature temperature sensors used on pulmonary artery catheters. The use of miniature thermistors is desirable because it allows for catheter sizes to be relatively small. One could of course reprogram the monitors, but such a solution to the problem would be costly and complicated, and may not be possible or practical in existing monitors. This invention provides an arrangement that allows a catheter-based temperature sensor to be connected to existing monitors.
An additional issue is that many patients, as their condition improves, do not require continuous monitoring of temperature, and therefore, do not require a dedicated connection between the catheter(s) and the monitor. At present, the dedicated connections limit how many patients the system can monitor, and increases the number of cables and connectors needed. It would be advantageous to free the system to allow monitoring more that one patient. This would, for example, enable nurse or physician to have a quick look at the patient""s temperature, possibly enter it into the patient""s chart, and then move on to other tasks or patients. It would therefore be beneficial to have an arrangement that provides this flexibility and simplicity. This invention does this as well.
In general, the invention provides an access device, such as a catheter, an introducer, or combination of catheters, introducers, probes and the like, that allows more accurate sensing of body temperature, for example, of a temperature medium such as blood, by insulating a temperature sensor from thermal contamination caused by a thermal mass, such as an infusion fluid or an instrument, introduced in portions of the access device. In the preferred embodiment of the invention the access device is a central venous device that includes a temperature sensor such as a thermistor, a thermocouple, etc.
The access device is insertable into the patient at a location of the temperature medium, and the access device includes at least one thermal mass other than the temperature medium. The access device supports the temperature sensor and includes at least one insulating structure insulating the temperature sensor from the thermal mass.
In certain embodiments of the invention, each thermal mass is located within a thermal lumen within the access device. The temperature sensor may be mounted externally to an outer surface of the access device, or within a sensor lumen of the access device. The insulating structure preferably extends between the temperature sensor and each thermal lumen.
The temperature sensor may also be mounted in or on a carrier. The insulating structure is then preferably formed as a barrier within the carrier and the carrier is held in one of the lumens of the access device with the barrier extending between the temperature sensor and the thermal lumen. The carrier may be removably insertable in the lumen of the access device.
In other embodiments of the invention, a pair of ports is formed in an outer wall of the access device and a flow channel is formed within the access device and extends between the pair of ports. The temperature medium, such as blood, then occupies the flow channel. The flow channel is located between the temperature sensor and the thermal lumen, or between the insulating structure and the thermal lumen, and thereby not only increases thermal contact between the temperature sensor and the temperature medium, but it also thermally isolates the temperature sensor further from the thermal lumen. The flow channel may thus itself form the insulating structure.
In another embodiment of the invention, the access device has an opening in an outer wall and the temperature sensor, when in a deployed position, extends into the opening. This increases thermal contact between the temperature sensor and the temperature medium and further insulates the temperature sensor from the thermal mass. If the temperature sensor is mounted on a carrier, then ends of the carrier may be secured within the access device. The carrier is then positioned between the temperature sensor and each thermal lumen, thereby forming the insulating structure.
The temperature sensor may alternatively be mounted within the carrier, which then protrudes as a loop out through the opening in the outer wall of the access device. The ends of the carrier are then preferably secured within the access device. In this embodiment, the insulating structure comprises a flow channel for the temperature medium, which is formed between the carrier and the access device at the position of the opening, and thus between the temperature sensor and the thermal mass. One advantage of this embodiment is that the temperature sensor is exposed substantially over its entire outer circumference to the temperature medium, via only the carrier.
Alternatively, the temperature sensor may be a right-angle thermistor mounted to extend out of the opening mainly perpendicular to a central axis of the access device.
In another embodiment of the invention, the temperature sensor is adhesively attached to the access device. The adhesive may be dissolvable at body temperature, so that the temperature sensor separates from contact with the access device when in position within the patient.
The access device may include a plurality of lumens, whereby the temperature sensor is mounted within a recess in an insulating member. The insulating member, together with the temperature sensor, are then mounted within one of the lumens of the access device so that the insulating member extends between the temperature sensor and the thermal lumen.
In another embodiment of the invention, the insulating structure includes an insulating material that is co-extruded with the access device and surrounds either at least a portion of each thermal lumen, or the temperature sensor itself.
In yet another embodiment of the invention, the access device has a lumen and a sensor port and the temperature sensor is mounted on a distal tip of a separate device, for example, a probe. The probe is insertable into the lumen of the access device so that the temperature sensor extends through the sensor port.
The insulating structure may also comprises a distal tip of the access device itself. The tip is then preferably formed from an insulating material as a separate member, and the temperature sensor is mounted within the distal tip. Alternatively, the distal tip of the access device may be provided with a lengthwise extending slit. The temperature sensor is then mounted on a first side of the distal tip and at least one thermal lumen carrying the thermal mass extends through a second side of the distal tip. The distal tip, in a deployed position, then separates along the slit, with the first and second sides of the tip being located on either side of the slit.
In another embodiment of the invention, the insulating structure is a lumen or a chamber in the access device that is expandable to increase the distance between the temperature sensor and the thermal mass.
The access device according to the invention is preferably included as a sensing member in a more general system for monitoring the body temperature of a patient. In this system, the access device is insertable into the patient and is connected to a temperature monitor that converts a sensor output signal of the access device into a patient temperature signal and for displaying the patient temperature signal. A connector is then provided to connect the temperature sensor with the temperature monitor.
The system according to the invention preferably further includes an adapter in the temperature monitor. The adapter converts the sensor output signal into a predetermined display format. The temperature monitor may also be provided with a display and a power supply, in which case the entire monitoring system may be implemented as a hand-held, self-contained unit that is portable between different patients.
The invention also encompasses a method for measuring the body temperature of the patient. The main steps of the method according to the invention involve supporting the temperature sensor on the access device; inserting the access device into a blood vessel; introducing at least one thermal mass into the access device; and insulating the temperature sensor from the thermal mass. In the preferred method according to the invention, the thermal mass is introduced via a thermal lumen located within the access device. One then mounts the temperature sensor in a sensor lumen within the access device and forms at least one thermally insulating structure between the temperature sensor and the thermal lumen. In some embodiments, to provide the thermally insulating structure, one may introduce a thermally insulating material into a lumen within the access device.
The invention also comprises a method for manufacturing the access device. In the preferred embodiment, this method comprises extruding the access device, forming a thermal lumen through which a thermal mass is introduced, forming a sensor lumen through which a temperature sensor is introduced, and forming an insulating structure separating the sensor lumen from the thermal mass. In manufacturing the access device, the temperature sensor may be mounted in the sensor lumen at a distal end of the access device. A signal wire is then drawn from the temperature sensor to an external patient monitor.